Effect of Gallic Acid Pretreatment and Application Mode on Dentin Bond Strength of a Universal Adhesive System After Thermal Aging: An In Vitro Study

Background: This in vitro study evaluated the effects of two different adhesive application approaches (total-etch and self-etch) and gallic acid (GA) pretreatment on the dentin microshear bond strength (μSBS) of a universal adhesive system. Bond strength was assessed both before thermal aging and following aging procedures simulating approximately 1 and 5 years of clinical service. Materials and Methods: One hundred twenty intact human incisors were allocated to experimental groups according to the adhesive strategy, presence or absence of gallic acid (GA) pretreatment, and thermocycling regimen (0, 10,000, or 50,000 cycles). A universal adhesive system (G-Premio BOND) in combination with a nanohybrid composite resin was applied in accordance with the manufacturers’ instructions. Microshear bond strength (µSBS) was determined using a universal testing device. The obtained data were analyzed by three-way ANOVA and subsequently compared using Tukey’s post hoc test at a significance level of 0.05. Results: In the total-etch approach, pretreatment with gallic acid (GA) resulted in significantly greater µSBS values than those observed in the corresponding untreated specimens under all aging conditions (no thermocycling: 18.53 ± 0.99 vs. 11.33 ± 0.81 MPa; 1-year: 19.86 ± 0.82 vs. 11.60 ± 0.58 MPa; 5-year: 19.04 ± 0.62 vs. 10.28 ± 0.83 MPa; p = 0.001). A comparable trend was noted for the self-etch strategy, where GA application significantly enhanced bond strength compared with the non-treated groups (no thermocycling: 21.70 ± 0.98 vs. 14.19 ± 1.17 MPa; 1-year: 22.60 ± 0.50 vs. 14.94 ± 0.85 MPa; 5-year: 22.32 ± 0.59 vs. 12.94 ± 0.84 MPa; p = 0.001). Across all thermocycling conditions, the self-etch mode consistently produced higher bond strength values than the total-etch mode. Thermal aging did not significantly influence µSBS in the GA-treated groups. In contrast, in the absence of GA pretreatment, thermocycling led to a reduction in bond strength, particularly after the 5-year aging protocol. Conclusions: Gallic acid pretreatment significantly improved dentin bond strength and contributed to the preservation of bond durability after thermal aging. The highest µSBS values were obtained when the self-etch approach was combined with gallic acid (GA) pretreatment, suggesting that GA may serve as a beneficial adjunct for improving the durability and long-term performance of resin–dentin bonds.